An optical fiber exhibits dispersion across a spectrum of light. This poses a severe limitation on a high-bit-rate data transmission by TDM or WDM techniques over a long distance optical fiber telecommunication line. Due to dispersion, every wave-length component of a light pulse starting from one end of a transmission line does not simultaneously arrive at the other end. When there occurs an excessive dispersion among the wave-length components, it becomes difficult to distinguish a front pulse from a rear one to process them.
One of ways for compensating dispersion is to introduce a delay-time-to-wave-length relationship, which is the reversal of the dispersion, into the transmission line at a relay point or a receiving point, by giving a greater delay time to a shorter wave-length component of light which runs faster in the transmission line and arrives earlier at the destination, and a smaller delay time to a longer wave-length component of light which runs slower and arrives later, to cancel differences in the arrival time among the wave-length components, viz the dispersion. As shown in FIG. 8, an optical circuit for compensating dispersion, comprising chirped Bragg gratings contained in an optical fiber 16 (hereinafter, a "chirped in-fiber Bragg grating" or "CFBG") and an optical circulator 17, has been proposed (Hill, K. O., et al, Optics Letters, 1994, 19, (17), pp 1314-1316, for instance).
In FIG. 8, a signal of light entering into a transmission line 18 from a source of light (not-illustrated) in the left, is subjected to dispersion in the long distance transmission line 18, and enters into the optical circulator 17 at an entrance a. The optical circulator 17 provides a relay point b to which the CFBG 16 is connected. In the CFBG 16, a multiplicity of Bragg gratings for reflecting a range of wave-length components of light are arranged, in which an interval between each pair of gratings (grating interval) varies continuously depending on the location of the gratings observed along the optical fiber. The compensated signal of light comes out at an exit c of the optical circulator 17.
One of wave-length components of light entering into the CFBG 16 from the relay point b is Bragg reflected in a region where the grating interval g satisfies: g=m.times.wave length of light in fiber/2 (m is an integer between 1-5, a diffraction mode), and returns to the relay point b. Other wave-length components having no connection with the above grating interval g simply pass through there. Gratings for reflecting (or resonant to) a longer wave-length component are placed nearer the relay point b, and those resonant to a shorter wave-length component, remoter. The fiber length between gratings for reflecting the maximum and minimum wave-length components is selected so that a round-trip time required by light for travelling that length is equal to the total dispersion time caused by the transmission line 18. A shorter wave-length component of light comes out from the CFBG 16 at the relay point b after consuming a greater delay time than that a longer wave-length component does in the CFBG 16. Thus, compensation of dispersion is attained in the signal of light coming-out at the exit c.
Now, suppose that dispersion suffered by a spectrum of light whose wave-lengths in vacuum are between 1.5 .mu.m and 1.6 .mu.m is compensated with the CFBG 16. Assuming m=4 and refractive index of glass=1.45, grating intervals for the wave lengths 1.6 .mu.m and 1.5 .mu.m are calculated to be g=2.207 .mu.m and g=2.069 .mu.m, respectively. Gratings having g=2.207 .mu.m are placed near the relay point b, those having g=2.069 .mu.m the other end, and those ranging between g=2.207 .mu.m and g=2.069 .mu.m are distributed in the middle of the CFBG 16.
The length of the transmission line whose dispersion can be compensated with the CFBG 16 is proportionate to the length thereof. The total dispersion time suffered by the above-mentioned 0.1 .mu.m spectrum of light in a 50 km transmission line, for example, is 85 ns (assuming that the rate of dispersion is 17 ps/nm/km), and the length of the CFBG having a round-trip time of 85 ns is 8.8 meters. If the length of the CFBG 16 is doubled to 17.6 meters, it becomes possible to compensate dispersion in a 100 km transmission line. However, to build the dispersion compensation circuit shown in FIG. 8, it is necessary to obtain a CFBG of as high precision and large length as described in the above.
FIGS. 9A and 9B illustrate conventional methods for producing a CFBG. In FIG. 9A, an optical fiber is in contact with a photomask having slits and irradiated with ultraviolet (UV) rays. In FIG. 9B, an optical fiber is continuously moved in the axial direction, and intermittently irradiated with UV rays. In either case, it is necessary to repeat positioning the optical fiber with a determined tension force, and irradiating the fiber with the UV rays. However, there has been a problem of stitching error in the gratings, in which the grating interval varies abruptly at places because of difficulty in precisely positioning the optical fiber every time. With such incomplete CFBG, a precise dispersion compensation can not be attained, due to irregular reflection caused by disorderly gratings. Another problem is difficulty in producing a long enough CFBG capable of compensating dispersion in a transmission line of 100 km class, because of repeated positioning and irradiation of the UV rays.